1. Field of the Invention
The present invention relates to chirp-controlled optical modulators, and to methods of controlling chirp in the modulation of optical signals, and is useful in particular, but not exclusively, for the modulation of optical signals in cable television (CATV) transmissions.
2. Description of the Related Art
It is well known in CATV systems to employ optic fibers for the transmission of television signals in the form of amplitude modulated vestigial side band optical signals. It is also well known to employ, as the source of the optical signals, a laser, for example a distributed feedback (DFB) semi-conductor laser, outputting an optical signal to an external optical modulator, for example an LiNbO.sub.3 Mach-Zehnder modulator.
A two stage Mach-Zehnder modulator has an electro-optic substrate on which there is provided an optical signal path comprising, in succession, an input wave guide, a splitter, a first pair of parallel phase modulation arms, a first coupler, a second pair of phase modulation arms, a second coupler and an output wave guide. The parallel phase modulation arms are associated with modulation electrodes, by means of which variable electric fields are applied across the modulation arms in order to correspondingly vary the refractive index of the electro-optical material of these arms and, thereby, to modulate an optical signal passing along the signal path.
It has become apparent by experimentation that there is significant composite second order (CSO) distortion generation with increasing fiber length in an analog optical fiber transmission system when using a prior art external optical modulator. The origin of this degradation has been confirmed, by experimental and theoretical considerations, to be the generation of chirp in the modulator.
It has previously been proposed to provide a single stage optical modulator with low chirp. Reference is made in this connection to Djupsjobacka "Residual Chirp in Integrated-optic Modulators", IEEE Photonics Technology Letters, Volume 4, No. 1, pp 41-43, Jan. 1992, according to which the dispersion effects of residual chirp arising from asymmetry in field overlap in Mach-Zehnder structures is analyzed and measured to indicate that a simple model for fiber mode propagation can be used together with an FFT-algorithm for calculating the effect of residual chirp from external modulators in multigigabit digital dispersive fiber-optic systems.